Scrap polyethylene terephthalate (PET) and PET production waste are often landfilled. Landfilling of these materials represents, among other things, a loss of raw material, and a potential ecological problem, if improperly landfilled. Accordingly, an economical process for the recycling of these materials is desirable.
The recycling of scrap PET and PET production waste, in general, is known. These materials can be reacted with methanol, i.e., "methanolysis", to produce dimethyl terephthalate (DMT). For example, see: U.S. Pat. Nos. 2,884,443; 3,073,050; 3,148,208; 3,321,510; 3,488,298; 3,907,868; 4,163,860; 4,578,502; 5,051,528; 5,414,106; and 5,414,107. These materials, alternatively, can be reacted with ethylene glycol, i.e., "glycolysis", to produce bis-(2-hydroxyethyl) terephthalate (BHET), a PET monomer. For example, see: U.S. Pat. No. 4,078,143, column 1. Scrap PET can, also, be melted and reformed without depolymerization. Additionally, there are known methods by which catalyst can be removed from PET production waste. For example, see: U.S. Pat. Nos. 4,013,519 and 4,118,582.
One problem that arises during methanolysis, results from the use of methanol containing water (e.g., methanol containing greater than 0.5% by weight water). Due to the presence of water, monomethyl terephthalate (MMT) and/or terephthalatic acid (TA) can be formed during methanolysis. Much of this MMT and TA acts as ligands for the catalyst, especially the manganese and antimony catalysts present in the stream. The ligands and metals combined together are known as coordination compounds.
Much of the coordination compounds is purged from the recycle process along with the bottoms from the glycol stripper column of the recycle process (explained below). The loss of MMT and TA coordination compounds is a yield loss, when the entire recycle process is considered.
Additionally, the coordination compounds that are not removed by the stripper column can foul downstream processing equipment, such as reboilers. The coordination compounds that foul the equipment form a viscous and difficult to handle material. This material causes equipment down-time for cleaning, and thereby increases costs. Therefore, the effective recovery of the coordination compounds could produce yield improvements to the overall process, as well, lowering costs by decreasing down-time for equipment cleaning.